For many years, manufacturers of sensitive electronic systems have recommended that users take measures to isolate their hardware from transient overvoltages (also called "surges") that may cause damage. Transient voltage protection systems (so-called "surge suppressors") are designed to reduce transient voltages to levels below hardware-damage susceptibility thresholds, which can be achieved through the use of various types of transient-suppressing elements coupled between the phase, neutral and/or ground conductors of an electrical distribution system.
Conventional transient-suppressing elements typically assume a high impedance state under normal operating voltages. When the voltage across a transient-suppressing element exceeds a predetermined threshold rating, however, the impedance of the element drops dramatically, essentially short-circuiting the electrical conductors and "shunting" the current associated with the transient voltage through the element and thus away from the sensitive electronic hardware to be protected.
To be reliable, a transient-suppressing element itself must be capable of handling many typical transient-voltage disturbances without internal degradation. This requirement dictates the use of heavy-duty components designed for the particular transient voltage environment in which such elements are to be used. In environments characterized by high-magnitude or frequently-occurring transients, multiple transient-suppressing elements may be required.
In many applications, the transient-suppressing elements typically employed are metal-oxide varistors ("MOVs"). When designing a system incorporating MOVs it is important to recognize the limitations of such devices, and the effects that the failure of any given MOV may have on the integrity of the total system. All MOV components have a maximum transient current rating; if the rating is exceeded, the MOV may fail. An MOV component may also fail if subjected to repeated operation, even if the maximum transient current rating is never exceeded. The number of repeated operations necessary to cause failure is a function of the magnitude of transient current conducted by an MOV during each operation: the lower the magnitude, the greater the number of operations necessary to cause failure. A designer of transient voltage protection systems must consider these electrical environment factors when selecting the number and type of MOVs to be used in a particular system.
Therefore, to design a reliable transient voltage suppression system, a designer must consider both the maximum single-pulse transient current to which the system may be subjected, as well as the possible frequency of transients having lower-level current characteristics. Although individual MOVs have a maximum transient current rating, it is possible to construct a device using multiple MOVs, in parallel combination, such that the MOVs share the total transient current. In this manner, each individual MOV must only conduct a fraction of the total transient current, thereby reducing the probability that any individual MOV will exceed its rated maximum transient current capacity. Furthermore, by using a plurality of individual MOVs, a transient voltage protection system can withstand a greater number of operations, because of the lower magnitude of transient current conducted by each individual MOV.
When a transient voltage suppression system incorporates multiple MOVS, it is important that the system be designed such that the failure of an individual MOV does not cause a complete loss of system functionality. When an MOV fails, due to either exceeding its maximum transient current rating or frequent operation, it initially falls into a low impedance state, drawing a large steady-state current from the electrical distribution system. This current, if not interrupted, will quickly drive an MOV into thermal runaway, typically resulting in an explosive failure of the MOV.
To avoid the explosive failure of MOVs, an appropriately-rated current-limiting element, such as a fuse, should be employed in series with MOVs. If the transient-suppressing device incorporates a plurality of MOVs, however, a single fuse in series with a parallel combination of MOVs may open-circuit even if only a single MOV fails, resulting in a disconnection of the remaining functional MOVs from the electrical distribution system. Therefore, better-designed systems incorporate individual fuses for each MOV, such that the failure of an individual MOV will result only in the opening of the fuse coupled in series with the failed MOV; the remaining functional MOVs remain connected to the electrical distribution system, via their own fuses, to provide continued transient voltage protection.
In the prior art there are circuits that incorporate a plurality of MOVs with an individual fuse provided for overcurrent protection of the MOVs, and with monitoring means provided to indicate the status of the fuse. U.S. Pat. No. 5,153,806 to Corey teaches the use of a single fuse to protect a plurality of MOVs, as well as an alarm circuit for indicating when the fuse has open-circuited. Similarly, U.S. Pat. No. 4,152,743 to Comstock teaches the use of a single fuse in series with a plurality of MOVs, as well as a light-emitting diode ("LED"), coupled in parallel with the fuse, to emit light when the fuse is blown.
The inadequacy of the prior art is that the failure of a single MOV component may cause the fuse in series with the plurality of MOVs to open-circuit, thus defeating the entire system and, therefore, all transient voltage suppression. Although the prior art teaches many methods of monitoring the status of an individual fuse, it fails to teach a method for monitoring the status of multiple fuses by means of a single display.
In U.S. Pat. No. 5,412,526, issued May 2, 1996, to Kapp, et al., a surge arrestor circuit having a plurality of MOVs with a fuse connected in series with each MOV is disclosed. The disclosed device further includes a circuit for "monitoring the status condition of the fuses." The circuit disclosed, however, employs only a single LED, the intensity of which "decreases slightly when a fuse opens up;" "when only a preselected number of varistor and fuse pairs remain operational, the LED! will extinguish entirely to signal for replacement of the surge arrestor." Thus, it is apparent that while the circuit disclosed by Kapp can indicate when a preselected number of varistors have failed, or fuses have open-circuited, it is incapable of indicating the remaining suppressing capacity of the device over the complete operational range of the device; i.e., over the range of zero to 100%. The circuit disclosed by Kapp, therefore, only provides a useful indication to a user if the preselected number of fuses have open-circuited; i.e., a user of the device cannot determine whether the suppressing capacity of the device is less than the full, original capacity until the preselected number of MOVs have failed. This is undesirable since a user cannot determine at what rate the MOVs within the device are failing, or how much suppressing capacity remains in the device until all, or substantially all, of the transient-voltage protection provided by the device is lost, at which time sensitive electronic systems to be protected by the device are subject to damage or failure due to subsequent transient voltages.
Therefore, what is needed in the art are circuits and methods for indicating the remaining suppressing capacity of a multiple-element transient-voltage protection device; the circuits and method should preferably provide an indication of the remaining suppressing capacity over the range of zero to 100%. Furthermore, there is a need in the art for circuits and methods that allow a user of a transient voltage protection device to determine the rate at which transient-suppressing elements within the device are failing, whereby the user can make an informed decision regarding the need for repair or replacement and whether the transient-voltage protection device should be replaced with a device having a greater capacity than the failed device.